This invention relates generally to optical add/drop multiplexers (OADMS) that may be used in wavelength division multiplexed networks to either add a channel or to drop a channel from the network.
Conventionally, optical networks may consist of carriers that carry a large number of channels, each channel being of a different wavelength. At stations along the network, additional channels may be added or channels may be dropped. Typically, an optical add/drop multiplexer is used to either add or withdraw such channels. The most conventional form of OADM includes a Mach-Zehnder interferometer including Bragg gratings.
The Mach-Zehnder interferometer with photo-induced Bragg gratings is an attractive device as a wavelength-selective OADM circuit. As an example, a Mach-Zehnder interferometer-based fiber grating may include identical Bragg gratings photo-imprinted in the two arms of a Mach-Zehnder interferometer. The Bragg gratings act as distributed-feedback reflection mirrors. A wavelength division multiplexed signal launched into the designated input port of the Mach-Zehnder interferometer is split evenly by a first 3 deciBel (dB) coupler, provided that the interferometer includes two 3 dB couplers having the same coupling ratio and the same arm path lengths.
The wavelength division multiplexed signal, except the Bragg-resonant wavelength, propagates along each arm to the second 3 dB coupler, where the wavelength division multiplexed signal is coherently recombined to emerge from the output port.
The signal of the Bragg-resonant wavelength is reflected back by the Bragg gratings located symmetrically in the two arms. The reflected Bragg-resonant wavelength appears from the drop port rather than the input port, because of the double half-a-n (n/2) phase shift arising at the 3 dB coupler. Owing to the merit of the symmetrical structure of the device, another signal of the Bragg wavelength inserted from the add port can be guided to the output port.
One problem with Bragg gratings is that, in some cases, they involve the use of sophisticated ultraviolet interference patterns and phase grating masks. The generation of these devices may be complex and their tuning can sometimes be awkward.
Thus, there is a need for an optical add/drop multiplexer with improved characteristics.